1. Field of the Invention
This invention relates to the generation of alternating current signals, and more particularly to the generation of alternating current signals the frequency of which changes over time.
One common device for providing such a signal is the sweep oscillator. A sweep oscillator produces an alternating current signal, the frequency of which is initially low. The frequency increases as time passes, ending at a high final frequency. The sweep oscillator then reverts to the initial starting frequency and the process is repeated over and over.
There are many applications wherein an electrical signal consisting of an alternating current of changing frequency, such as is provided by a sweep oscillator, is needed. One such application is as the source of the reference frequencies in a frequency spectrum analyzer. The function of a spectrum analyzer is to determine the frequency or frequencies present in an unknown signal. Typically, the unknown signal will consist of a plurality of different frequencies mixed together, and it is the job of the spectrum analyzer to determine which frequencies are present in the unknown signal.
A spectrum analyzer works by electronically mixing, or heterodyning, a reference signal with the unknown signal. The output of said mixing process is a new signal with a frequency equal to the difference between the frequencies of the signals being mixed. This new signal is applied to a narrow band pass filter. The filter produces an output only if the input signal is of the same frequency as that to which the filter is tuned; signals of all other frequencies are blocked by the filter. The output from the filter is displayed on the screen of an oscilloscope (or some other viewing device). Thus, the oscilloscope will show the presence of a signal only if the signal present at the input to the filter is of the same frequency as the frequency to which the filter is tuned.
When the oscilloscope indcates the presence of an output from the filter, the frequency of the unknown input signal can be computed simply by adding the numerical value of the frequency of the reference signal to the value of the frequency to which the filter is tuned. By applying enough different reference signals to the mixer, and observing which reference frequencies result in an indication from the oscilloscope, it is possible to determine all of the different frequencies present in the unknown signal. However, the accuracy of this determination is limited by the precision with which the frequency of the reference signals is known.
In a spectrum analyzer, the reference signals are generated by a sweep generator. The swe generator generates all frequencies between its minimum and maximum many times per second. The accuracy of the output determination from the spectrum analyzer at any given instant of time will depend on how precisely the frequency being produced by the sweep generator at that instant is known.
The spectrum analyzer is but one of many applications which require a sweep generator the output of which is accurately known at all times during the sweep. Hence there is a need for a sweep oscillator, the output frequency of which is precisely known at all times during the sweep.
2. The Prior Art
Techniques for accurately controlling the frequency of a fixed-frequency oscillator are well known. Examples of such techniques include: the use of a crystal as the frequency determining element; the use of temperature stabilizing techniques in the oscillator circuit; the use of highprecision parts in the oscillator circuit; and the use of phase-locked loops.
Unfortunately, such techniques have not been sufficient to create a sweep oscillator of the accuracy needed. A crystal can only be used in a fixed-frequency oscillator. A phase-locked loop cannot change frequencies fast enough to generate a rapid sweep. High-precision parts and temperature stabilization techniques can be used in sweep oscillators, and they are so used but, by themselves, these methods are not enough to achieve the desired accuracy.
It is possible to accurately determine the frequency at any one point in the output of a sweep generator. One method is to use the output from an accurately calibrated fixed-frequency oscillator to fix the mid-frequency of the sweep oscillator. Another method is to superimpose the accurately calibrated output from a marker generator onto the unknown signal being analyzed. However, these methods provide precise knowledge of the output frequency of the sweep oscillator at only one, or at most a few, of the many output frequencies in the range of its sweep. Hence, the need for a sweep oscillator, the output frequency of which is precisely known at all times during the sweep, has not yet been satisfied.